Mathematical fact: 2637 is the number of commutative monoids of order 7.
Trivia: 2637 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Historical date: March 21st is the day in 1943 that Wehrmacht officer Rudolf Christoph Freiherr von Gersdorff plots to assassinate Adolf Hitler by using a suicide bomb, but the plan falls through.